The background of this invention is partly represented by U.S. Pat. Nos. 3,969,474, 3,000,237, 3,063,349, which disclose methods for forming steel cutting rules for either reciprocating flat bed, or rotary cylindrical press apparatus. In addition, the state of the art has progressed to include dies that are prepared by etching magnetic metals, such as spring steel or magnetic stainless steel. The etched dies are attached in rotary press lines to permanent magnet surfaced cylinders that are rotated in synchrony with other elements of the press line such as printing plate cylinders, glue applicators, etc.
The etching of dies is accomplished primarily by the use of photographic techniques in which the metal is first covered by a photosensitive lacquer or solid film etchant resistive material, which is then exposed to actinic rays through a photographic transparency containing an opaque image of the features of the die's cutting or forming surfaces. Etched magnetic dies are increasingly used in the printing, and packaging industries, not only because of the accuracy, reproducibility and the efficiency of etched die production, but also because etched dies are much more quickly prepared, since neither preparation of machined parts, bending with special tools, nor the fabrication of molds are required as they are in earlier steel rule die methods or in the technique disclosed in U.S. Pat. No. 3,969,474. U.S. Pat. No. 3,969,474 primarily addresses molded steel rule dies for rotary die cutting.
Speed of die preparation is of great importance to printing firms who often face operating deadlines with little detailed information until near press time. The photographic transparencies used to produce etched dies can be produced from computer aided design (CAD) which can be downloaded by modem to plotting devices which optically expose the pattern directly onto the photographic film. This technique further reduces the lead time required to produce etched dies.
Despite the several advantages of etched magnetic dies, the use has been limited primarily by the cost of magnetic cylinders. In typical magnetic cylinders, magnets of high magnetic strength, such as costly samarium-cobalt or neodymium cobalt permanent magnets, are closely arranged and permanently attached around a stainless steel cylindrical drum surface so that the etched dies may be placed anywhere on the surface and be firmly held in place so as not to shift during the actual cutting operation as paper or film passes between the cutting edge of the die on the rotating cylinder and anvils of various materials.
The use of magnetic dies has been even further limited by the economic consideration of the large number of already installed non-magnetic cylinders in rotary press lines and the familiarity of press operators with the methods of mounting molded dies to these cylinders. These non-magnetic cylinders typically contain a large plurality of tapped holes located on a regular pitch which may be used to bolt thin clamps which are used to hold molded dies in position utilizing ledges that are molded into the molded plastic portion of the dies.
It is therefore an object of the invention to provide an apparatus and means for the practical use of magnetic dies on non-magnetic cylinders.
Another object of the invention is to provide an apparatus and means for preventing slippage between magnetic dies and magnetic cylinders in the operating mode.